In this paper, first, the surface potential of thin-film transistors with light-doped polysilicon is analyzed. Next, the gate voltage, which corresponds to the channel current that quickly increases when the surface potential deviates from the sub-threshold region, is considered as the threshold voltage. Then, by taking into consideration the single-exponential distribution of trap state density, the surface potential of the transistor is derived, and an ana- lytical model of the threshold voltage is presented. Finally, a numerical simulation is performed to verify the model. The results indicate that the threshold voltage obtained by the proposed model perfectly matches that extracted by the second-derivative method.